There is a need in the food packaging industry for packing film with improved structural integrity and which can be made into bags. Food products such as cheese, processed meat and raw meat are preferably packed by using film with high barrier resistance to moisture and oxygen. A typical way of protecting such food products from moisture and oxygen is by shrink packaging in high barrier film. A useful film with good moisture and oxygen barrier properties for this purposes comprises a copolymer of vinylidene chloride with a monomer such as methyl acrylate, vinyl chloride or other unsaturated monomer or hydrolysed ethylene vinyl acetate (EVOH) or any other barrier polymer which can be fabricated in the form of film and has good oxygen and moisture barrier properties.
It is known to provide a multilayer shrink film with the following structure, namely a first outer layer, a bonding layer, a barrier layer, a further bonding layer and a further outer layer to achieve the desired properties. A known multilayer film of this kind having good moisture and oxygen barrier properties for use in packaging food products has a core layer of a barrier polymer such as a copolymer of vinylidene chloride or hydrolysed ethylene vinyl acetate (EVOH) or other barrier polymer. A multilayer plastic film from which bags are to be manufactured must meet a number of criteria, such as being heat sealable, delamination resistant, and have good moisture and oxygen barrier properties. For heat shrinkable uses, the film must also be capable of shrinking from about 30 to about 50 percent at 90 degrees C. in both longitudinal and transverse directions. The film should also be capable of withstanding physical abuse during transportation.
A significant property of EVOH and polyvinylidene chloride (PVDC) based films is their barrier resistance to oxygen and other gasses. Compared to PVDC based films, EVOH based films have better barrier properties at low relative humidity, i.e. 30 to 40 percent, than commonly used multilayer films.
U.S. Pat. No. 4,469,742 (Oberle et al) describes a multilayer cook-in film with six layers. The middle barrier layer is of EVOH, selected because of its superior oxygen barrier properties compared to PVDC.
However, EVOH is hydrophillic, and therefore EVOH based films are moisture sensitive. As the moisture content of an EVOH based film increases, the permeation rate increases, i.e. barrier properties decrease, which is in contrast to PVDC based films because the permeability of PVDC does not vary with humidity, i.e. PVDC is not moisture sensitive. Accordingly therefore, EVOH based films are often produced in a multilayer structure or buried under moisture barrier materials such as polyolefins or sometimes with polyamides in order to protect an EVOH layer from moisture.
If a material with a much higher barrier layer for moisture compared to polyolefins or polyamide is used to protect an EVOH layer in a multilayer film from both the environmental and product sides thereof, the barrier properties and consequently the shelf life of the product can be increased significantly.
However, U.S. Pat. No. 4,161,562 (Yoshikawa et al) teaches that, in multilayer barrier films, a barrier layer of crystalline polymers such PVDC or EVOH has a tendency to crack if the thickness exceeds 15 microns, due to low temperature brittleness of these polymers.
It is therefore an object of the present invention to provide a multilayer plastic film which is at least substantially free from the problems referred to above.